1. Technical Field
The present invention relates to control circuits for controlling light emitting devices.
2. Related Art
Lighting systems have been developed in which light emitting diodes (LEDs) are used as light emitting devices for use in lighting.
FIG. 11 shows a control circuit 100 for a conventional lighting system. The control circuit 100 has a configuration in which a rectifier unit 10, a rectifier capacitor 12, a choke coil 14, a regenerative diode 16, a switching element 18, a control unit 20, and a comparator 22 are contained.
When AC power is supplied to the rectifier unit 10, full wave rectification is applied to the AC power. A full wave rectified voltage is smoothed by the rectifier capacitor 12, and subsequently supplied to the control unit 20 as a source voltage, and to an anode terminal of an LED 102 as a drive voltage. A cathode of the LED 102 is connected via a series connection of the choke coil 14, the switching element 18, and a resistance element R1 to ground. In response to switching operation of the switching element 18 controlled by the control unit 20, an electric current is passed, via the choke coil 14, the switching element 18, and the resistance element R1, through the LED 102, thereby causing the LED 102 to emit light. Further, the regenerative diode 16, which transfers energy having been accumulated in the choke coil 14 to the LED 102 for regenerative use when the switching element 18 is turned off, is connected in parallel to the LED 102 and the choke coil 14.
The comparator 22 receives input of both a comparison voltage Vcmp generated across the resistance element R1 by the current flowing through the LED 102 and a fixed reference voltage Vref obtained by dividing, among resistances, a voltage Vreg generated in the control unit 20 that has received the smoothed power. The control unit 20 controls switching of the switching element 18 based on a result of comparison between the reference voltage Vref and the comparison voltage Vcmp performed in the comparator 22. The control unit 20 turns on the switching element 18 to allow a flow of the current through the LED 102 when the comparison voltage Vcmp is lower than the reference voltage Vref, and turns off the switching element 18 to interrupt the current to the LED 102 when the comparison voltage Vcmp becomes higher than the reference voltage Vref.
As described above, an average emission intensity of the LED 102 can be adjusted by controlling the flow of the current through the LED 102.
However, the above-described control circuit 100 in related art suffers from a problem that it is not possible to increase a power factor because, as shown in FIG. 12, the AC voltage to be input and the current flowing through the LED 102 are out of phase due to the fact that the reference voltage Vref is a constant voltage.
Meanwhile, dimmer systems for incandescent light bulbs in which the emission intensity (brightness) can be adjusted have been utilized. In the dimmer systems for the incandescent light bulbs, as shown in FIG. 13, the emission intensity is adjusted by controlling a conduction angle of AC power so as to reduce an average value of the current flowing through the incandescent light bulb.
On the other hand, there has been a desire for a system which is capable of adjusting the emission intensity also in a case where an LED is used as the light emitting device. Conventionally, a processing circuit for converting an alternating current voltage into a digital voltage signal and a circuit for detecting a time when the alternating current voltage is shut off and stopping oscillation of an inverter at the detected time are used in the dimmer system for the LED.
However, it is necessary to install the above-described circuits as different circuits independent of the system for the incandescent light bulbs which has been conventionally provided as facilities of an accommodation unit. In addition, each of the circuits is relatively large in size. For this reason, the circuits used as a control system for the LED have problems such as increased manufacturing costs.
Therefore, it is desired to provide a control circuit capable of adjusting light intensity of an LED by means of a conventional light intensity adjusting circuit designed for the incandescent light bulbs.
Further, the conventional light intensity adjusting circuit for the incandescent light bulbs has a different minimum output voltage for each manufacturer. In other words, control ranges of the conduction angle of the alternating current voltage differ among the light intensity adjusting circuits, which results in mixed presence of the light intensity adjusting circuits such as those having a minimum output voltage of 30 V, or those having a minimum output voltage of 60 V.
For example, it is assumed that a control circuit for controlling switching of an LED is configured so as to match a voltage adjustable range in a light intensity adjusting circuit whose minimum output voltage is 30 V (i.e. the voltage adjustable range of from 30 V to a maximum output voltage). If the control circuit is applied to another light intensity adjusting circuit whose minimum output voltage is 60 V, in spite of the fact that light intensity of the LED can be adjusted at voltages in a range of from 30 V to 60 V with the control circuit, the voltages in the range are unavailable in the light intensity adjusting circuit, which results in a problem that light produced by the LED cannot be adjusted to a state of minimum light intensity (a darkest state), or the like. On the other hand, when the control circuit for controlling switching of the LED is configured so as to match a voltage adjustable range in a light intensity adjusting circuit whose minimum output voltage is 60 V (i.e. the voltage adjustable range of from 60 V to the maximum output voltage), application of the control circuit to another light intensity adjusting circuit whose minimum output voltage is 30 V brings about a problem that switching control performed by the control circuit becomes unstable at voltages ranging from 30 V to 60 V.
In this respect, it is also desired to provide a control circuit capable of adjusting light of an LED to the state of minimum light intensity (the darkest state) regardless of which light intensity adjusting circuit is used, and regardless of the minimum output voltage of the light intensity adjusting circuit.